The present invention relates to an electrohydraulic control system for controlling a hydraulic working cylinder, for example of a press or the like, the piston of which has a main working surface and a smaller working surface on the rod side, with each working surface delimiting a pressure chamber, and with the piston being able to carry out a rapid feed movement or a loading movement in one direction and a rapid retracting movement in the opposite direction; mechanically operated control valves are provided for charging the pressure chambers with pressure.
In a hydraulic press, the movement of the press tool is produced with a hydraulic cylinder. The press tool is generally fastened to the free end of the piston of the hydraulic cylinder. At the commencement of a work cycle, the piston is brought as close as possible to the workpiece at high speed and relatively low feed force, and is then moved on at increased feed force and reduced speed. If a punch press is involved, the working feed movement ends with the ejection of the workpiece that is being punched or stamped out. After termination of the working feed movement, the piston is brought back into its initial position at high speed.
The heretofore known systems that are used for controlling such hydraulic drives are relatively complicated. Here, special difficulties arise, on the one hend with the variation in the speed of the working piston and the variation in the feed force linked therewith, and on the other hand with the reversal of the direction of movement of the piston. Path-dependent control systems used in the beginning have not proved to be satisfactory. Besides the main disadvantages of inadequate accuracy of response and the risk of variations during operation, relatively long work cycle time intervals have to be reckoned with, which of necessity lead to a correspondingly unfavorable utilization of a press or the like.
In another known control system of a hydraulic drive means, the switching-over from rapid to load feed operation occurs in a pressure-dependent manner. Here, among other things, electromagnetic pressure switches are provided that respond to the pressure-charged pressure chamber of the working cylinder and bring about charging of the pressure chamber with pressure by means of electrical signals, when required. Auxiliary equipment is necessary here for reversing the direction of the piston, so that in the case of such a control system the overall technical expenditure is very considerable. In other respects, the cycle times are also relatively great in such a control system.
It is therefore an object of the present invention to provide an improved system for controlling a hydraulic working cylinder in such a way that the disadvantages of known systems do not arise, i.e., the inventive system must be such that with a favorable power balance, the switching-over of the working piston can be carried out in a problem-free manner with regard to speed, feed pressure, and direction of movement, that the working piston can initiate each position within its stroke, that optimally short work cycle times can be achieved, and that the necessary hydraulic controls can be accomplished with simple control valves that are reliable in operation. With regard to the attainment of an advantageous power balance, only that amount of energy is intended to be used with the idle strokes so that the occurring friction is overcome and the necessary mass acceleration can be realized.